1. Field
The inventive concept relates to a distributed antenna system (DAS), and more particularly, to an uplink signal transmission operation of each of node units constituting a DAS.
2. Description of Related Art
Base stations are established at proper positions so as to maintain the quality of mobile communication services, but much cost is required to establish and operate base stations. Therefore, separate communication relay systems for mobile communication relay are built up in shadow areas such as in-buildings, subways, underground roadways, and tunnels, which are locally generated due to electromagnetic interference or physical barriers. A representative among the communication relay systems is a distributed antenna system (DAS).
In the DAS, a plurality of remote units (RUs) each including an antenna are evenly distributed in mobile communication service areas, thereby providing improved mobile communication services. The DAS may include a master unit (MU) connected to a base station (BTS) and a plurality of RUs connected to the MU. The RUs evenly distributed in all of the areas form virtual cells with a user's mobile communication terminal, to provide high-quality communication services to even mobile communication terminals located at boundaries between the cells.
Each RU of the DAS performs mobile communication with the BTS through the MU. In a downlink (forward direction) in which a signal is transmitted from the BTS to a mobile communication terminal, the BTS transmits a downlink signal to the MU, and the MU transmits the downlink signal received from the BTS to each RU such that the downlink signal is transmitted to the mobile communication terminal. In an uplink (reverse direction) in which a signal is transmitted from the mobile communication terminal to the BTS, the mobile communication terminal transmits an uplink signal to an RU connected thereto, the RU transmits the received uplink signal to an RU as an upper node (node connected closer to the MU), and the RU as the upper node transmits the uplink signal to the MU such that the uplink signal is transmitted to the BTS.
However, since the MU and the BTS are connected through one channel in the DAS, the MU should transmit, through one path, uplink signals received from the plurality of RUs connected thereto. To this end, a conventional method was used in which the MU arithmetically summed a plurality of digital uplink signals. For example, it is assumed that the MU has received two digital uplink signals, and one of the two digital uplink signals correspond to [1 2 3] and the other of the two digital uplink signals correspond to [4 5 6]. In this case, according to the conventional method, the MU may generate a summed uplink signal corresponding to [5 7 9] by arithmetically summing the two digital uplink signals. A function of preventing digital overflow may be additionally performed on the generated summed uplink signal.
In the conventional method, a transmission target uplink signal, which is transmitted to upper node units, can be simply generated by arithmetically summing received uplink signals. However, the magnitude of the transmission target uplink signal increases, and therefore, digital overflow may occur. In order to solve this problem, a separate auto level control (ALC) function should be performed on the transmission target uplink signal, which results in an increased in complexity of the DAS.